1. Field of the Invention
The present invention relates to an intermediate transfer medium for use in electrophotographic image formation, and to an image forming apparatus using the intermediate transfer medium.
2. Discussion of the Background
Recently, various electrophotographic full color image forming apparatus have been commercialized. Among the full color image forming apparatus, double-transfer type image forming methods (hereinafter referred to as intermediate transfer methods) in which color toner images such as yellow, magenta, cyan and black images formed on an image bearing member or plural image bearing members are primarily transferred on an intermediate transfer medium one by one so as to be overlaid, resulting in formation of a full color toner image, and the full color toner image is then secondly transferred to a receiving material are typically used because of having advantages such that images can be formed on various kinds of receiving materials and double-sided copies can be easily produced.
In the image forming apparatus using the intermediate transfer method, various image forming members such as photoreceptors, second transfer rollers, cleaning blades, etc. are contacted with or rubbed by the surface of the intermediate transfer medium while optionally receiving electric stimulation. Therefore, the intermediate transfer medium is required to have good mechanical and electrical resistance. Specifically, the intermediate transfer medium preferably has the following properties:    (1) good abrasion resistance;    (2) good toner releasability;    (3) good surface properties (e.g., friction coefficient); and    (4) good mechanical properties (e.g., linear elasticity and elongation rate).
It has been proposed in published unexamined Japanese Patent Applications Nos. (hereinafter referred to as JP-As) 05-46035, 08-30117, 09-269676, 10-20538 and 11-231678 to use silicone resins and elastomers for an intermediate transfer medium to impart good toner releasability thereto. JP-As 05-311016 and 07-24912 have disclosed polyolefin type intermediate transfer media. However, it is difficult for the techniques to improve the resistance of the intermediate transfer media to abrasion and scratch.
JP-As 05-40417, 06-234903, 07-92825, 08-267605 and 10-166508 have disclosed to use fluorine-containing resins and elastomers for intermediate transfer media to impart good toner releasability thereto. However, the technique has a drawback in that when such materials are used for the outermost layer, the adhesion between the layer and the substrate of the intermediate transfer media deteriorates, resulting in occurrence of a problem in that the outmost layer peels from the substrate.
In addition, other materials such as polycarbonate materials (disclosed in JP-As 06-93175, 06-149081, 06-14908310-10880 and 0.13-31849); polyester materials (disclosed in JP-As 13-13801 and 13-18284); polyurethane materials (disclosed in JP-As 10-319727 and 11-30915); polyimide materials (disclosed in JP-As 07-156287, 08-176319, 11-24427, 11-170389 and 12-172085); and polyimide materials in which a particulate fluorine-containing resin or a fluorine-containing organic compound is dispersed (JP-A 11-156971, 11-119560 and 07-156287), have been proposed. However, the toner releasability of these intermediate transfer media is unsatisfactory. Specifically, a problem in that toner particles and additives of the toner adhere to the surface of the intermediate transfer media tends to occur.
Image forming apparatus using an intermediate transfer method tend to cause a problem in that abnormal images are formed in the primary and second transfer processes depending on the transfer rate of the toner used.
One of the abnormal images is toner scattering. Specifically, the toner scattering problem is such that toner particles in an image are scattered in the primary and/or second transfer processes. More specifically, the toner scattering problem is such that a toner image formed on a photoreceptor is not transferred to a desired position of an intermediate transfer medium and the transferred toner particles are scattered thereabout, resulting in blurring of the resultant toner image and deterioration of sharpness of fine line images.
In attempting to solve such a toner scattering problem, JP-A 63-34570 discloses an image forming method in which an image formed of a toner having a high electric resistance is non-electrostatically transferred onto an intermediate transfer medium, and then the toner image is transferred/fixed to a receiving material using a heat roller upon application of heat and pressure thereto. In addition, JP-A 63-34571 discloses an image forming method in which an image formed of an electroconductive toner is non-electrostatically transferred onto an intermediate transfer medium, and then the toner image is transferred/fixed to a receiving material using a heat roller upon application of heat and pressure thereto. However, these methods have a drawback in that the receiving paper has to be a special receiving material having a high heat resistance so as not to cause a problem when heated by a heat roller. Therefore, the methods do not have the above-mentioned paper-free advantage of the intermediate transfer method.
In attempting to solve the toner scattering problem, JP-A 01-282571 discloses a color image forming apparatus in which whenever toner images are transferred to an intermediate transfer medium, the toner images are discharged using a paper separating charger. In addition, JP-A 02-183276 discloses a full color image forming apparatus in which an intermediate transfer medium is charged so that the transfer voltage in a final color toner image transfer process is higher than that the last color image transfer process in which another color toner image is transferred. Further, JP-A 04-147170 discloses an image forming apparatus in which an intermediate transfer medium is discharged before the toner image on the intermediate transfer medium is transferred on a receiving paper. However, these apparatus have to be equipped with a discharging means or a voltage applicator, and thereby the apparatus become complex in structure and become large in size.
Another of the abnormal images caused when a toner image is transferred is omission. Specifically the omission problem is such that a toner image transferred on an intermediate transfer medium has a pin point omission, wherein the toner image corresponding to the omission remains on the intermediate transfer medium without being transferred.
In attempting to solve this omission problem, JP-A 58-187968 discloses a toner image transfer device in which an organic fluorine-containing compound is applied to the surface of the intermediate transfer medium thereof. In addition, JP-A 02-198476 discloses an image forming apparatus using an intermediate transfer medium including a wettability controlling agent therein. Further, JP-A 02-213881 discloses an image forming apparatus using an intermediate transfer medium having an outermost layer including a lubricant such as zinc stearate. Furthermore, JP-A 03-242667 discloses an image forming apparatus using an intermediate transfer medium which is made of a silicone rubber and which has a surface having a specific surface roughness. JP-A 04-305666 discloses an image forming apparatus in which a contact member which rotates at a speed different from that of the intermediate transfer medium to rub the surface of the intermediate transfer medium. In addition, JP-A 05-307344 discloses an image forming apparatus in which the surface of the intermediate transfer medium thereof is ground when a toner film is formed on the intermediate transfer medium. Further, JP-A 05-313526 discloses an image forming apparatus including a member for rubbing the surface of the intermediate transfer medium thereof if desired after checking the surface roughness of the surface. JP-A 05-323802 discloses an image forming apparatus in which the intermediate transfer medium thereof is rubbed after the predetermined number of copies are produced. However, the omission problem can not be perfectly solved by these techniques.
Because of these reasons, a need exists for an image forming apparatus which uses an intermediate transfer medium and which can produce high quality images without abnormal images such as toner scattering and omission.